Ferroelectricity is a very general phenomenon. Yet it is possible to divide it into three main groups with essentially three different mechanisms: the order-disorder of hydrogen bonds, the trigger effect of nonspherical radicals, and the 4π/3 catastrophe due to high electronic polarizabilities. Recently it has been realized that some of the metallic bronzes crystallizing in the perovskite structure may also be ferroelectric. At present, it is very difficult to determine ferroelectricity in these metals.